Light-emitting diodes (LEDs) and like semiconductor light-emitting elements have advantages of being compact, low power consuming, and being capable of stable, high-luminance light emission. Lighting fixtures that use light-emitting devices provided with white-light-emitting LEDs have progressively replaced incandescent lamps and other lighting fixtures. Examples of the white-light-emitting LED include combinations of blue LEDs and YAG phosphor (composition being (Y,Gd)3(Al,Ga)5O12).
The light-emitting device thus configured produces white light by mixing the blue light from the LED with the yellow light emitted from the YAG phosphor. The configuration is short on emission of red light due to the light-emitting properties of the YAG phosphor, and thus is not ideal for emission of warm white light that resembles the incandescent bulb color, which warm white light is in the need for home lighting fixtures and other applications.
Disclosed in view of these issues is a light-emitting device capable of emitting reddish warm white light by combining a blue LED, YAG phosphor, and an additional nitride-based red phosphor (see, for example, Patent Literature 1).
A light-emitting device, configured as exemplified in Patent Literature 1, allows for emitting white light that exhibits a large value for a color rendering index (Ra) at color temperatures in the incandescent bulb color range, of 3,250 K or lower, and especially can emit white light exhibiting an excellent value for a special color rendering index (R9), which R9 is a measure of how red is rendered.
However, according to the configuration, the red phosphor absorbs fluorescent light emitted by the YAG phosphor. In this manner, a large proportion of light emitted by one phosphor is absorbed by another phosphor (mutual light absorption of phosphors), thereby greatly reducing light emission efficiency of the light-emitting device.
Under these circumstances, a configuration is disclosed which holds down mutual light absorption of phosphors (see, for example, Patent Literature 2). The configuration includes a blue LED, a green phosphor, and a yellow thru/or a red phosphor. The green phosphor emits light with a peak wavelength in a range from at least 500 nm to less than 540 nm. The yellow thru/or a red phosphor emits light having a peak wavelength in a range from 575 nm to 650 nm and also having, at a wavelength of 520 nm of the excitation spectrum, an intensity equal to or less than 60% of the intensity at the peak wavelength of the excitation spectrum.
Japanese Patent Application Publication, Tokukai, No. 2003-321675A (Published Nov. 14, 2003)
Japanese Patent Application Publication, Tokukai, No. 2008-244468A (Published Oct. 9, 2008)